A southwest-facing roof with a 25° pitch does not produce the same amount of electricity as a south-facing roof with a 35° pitch. Before sizing a photovoltaic system, you need reliable figures to compare these configurations. PVGIS, the European Commission’s free tool, lets you estimate solar output based on precise parameters such as orientation, tilt and location. But you need to know how to interpret its results and understand its limitations.
Default losses in PVGIS: a setting you shouldn’t overlook
When you run a simulation in PVGIS, the tool automatically applies a loss rate. This parameter covers several factors: electrical losses in the wiring, dirt on the modules, the effects of snow and ageing. The default setting puts these losses at 14% for a grid-connected system.
This figure matters. If you compare two orientations (south vs. southeast, for example) without changing this setting, the production difference shown includes the same 14% losses for both. The result therefore does not measure the effect of orientation alone.
To isolate the impact of azimuth or tilt, you can adjust this parameter manually. An installer familiar with the planned equipment (inverter, cable cross-section, mounting type) can fine-tune this value.
Comparing two simulations with losses tailored to each configuration gives you a more realistic production difference than simply copying the standard setting. The orientation of photovoltaic solar panels has a significant effect on annual output, but you can only assess this difference accurately if the other parameters are also controlled.
PVGIS version 5.3: how satellite data changes solar estimates
PVGIS version 5.3, currently in production, uses different datasets depending on the geographical area. For Europe, the model uses SARAH-3, which offers finer spatial resolution than ERA5. In practice, SARAH-3’s finer grid improves how sunlight is represented in areas where terrain or climate conditions vary over short distances.
This improvement in accuracy has a practical limitation. The data is still gridded: a steep-sided valley, a partially shaded north-facing slope or an urban microclimate may be smoothed out by the model. PVGIS cannot see the trees growing in front of a roof or the neighbour’s chimney.
No major change between versions 5.2 and 5.3
A comparison by the JRC (Joint Research Centre) using reference stations shows a very small average difference between the results from the two versions. So upgrading to the latest version does not guarantee a radically different result. Your choice of input parameters matters more than the version number.
In short, updating your PVGIS simulation to version 5.3 is good practice, but setting the tilt, azimuth and losses to realistic values has a greater impact on the reliability of the result.
Comparing orientations with PVGIS: hourly rather than annual output
Most users look at the annual total in kWh. This is a logical first step, but it is not enough for a self-consumption project. Two orientations can show similar annual totals while generating electricity at very different times.
PVGIS provides hourly results, which is where the comparison becomes useful. A southeast-facing system produces more in the morning, while a southwest-facing system shifts the peak to the afternoon. For a household that uses most of its electricity in the late afternoon or evening (oven, washing machine, vehicle charging), a west-facing orientation can make more kWh available for self-consumption than a due-south orientation, even if its annual output is lower.
How to use hourly data
- Run two PVGIS simulations with different azimuths (for example, 0° for south and -45° for southeast), keeping the tilt and losses the same
- Download the hourly results in CSV format from the tool’s dedicated tab
- Overlay the production curves with the household or building’s consumption profile, hour by hour
- Identify the time slots when production actually covers consumption and when surplus electricity is exported to the grid
This approach takes a little more work than simply checking the annual total. However, it helps you avoid oversizing a system or choosing an orientation that generates plenty of electricity, but at the wrong time.
Azimuth and tilt in PVGIS: parameters that affect photovoltaic output
In the PVGIS interface, azimuth is set in degrees relative to south (0° = due south, -90° = east, 90° = west). Tilt is the angle of the panel relative to the horizontal. Together, these two parameters determine how much solar energy is captured throughout the seasons.
In mainland France, the right tilt depends on latitude and consumption patterns. A steeper panel captures more energy in winter, when the sun is low, while a shallower tilt favours summer production. PVGIS also offers an “optimal tilt” function that calculates the angle that maximizes annual output for a given azimuth.
Be careful not to rely solely on this automatic optimization. It increases annual output, not financial returns. If the feed-in tariff or the share of self-consumption varies by time of day, the tilt that PVGIS considers optimal is not necessarily the most profitable.
What PVGIS does not model
- Nearby shading obstacles: trees, neighbouring buildings, antennas and chimneys
- The actual degradation of modules over time, which depends on the manufacturer and local conditions
- Losses caused by an undersized inverter or unsuitable wiring
PVGIS is an estimation tool, not an on-site audit. Combining its results with a technical site visit that records shading obstacles and assesses the roof helps turn a theoretical estimate into reliable system sizing. The simulator provides a solid starting point, as long as you do not treat its results as a definitive measurement.
